In recent years, the telecommunications industry has witnessed the proliferation of a variety of digital vocoders in order to meet bandwidth demands of different wireline and wireless communication systems. The name &lt;&lt;vocoder&gt;&gt; stems from the fact that its applications are specific to the encoding and decoding of voice signals primarily. Vocoders are usually integrated in mobile telephones and the base stations of the communication network. They provide speech compression of a digitized voice signal as well as the reverse transformation. Typically, a voice signal is digitized through one of many quantization techniques. Examples of these techniques are Pulse Amplitude Modulation (PAM), Pulse Code Modulation (PCM) and Delta Modulation. For the purposes of this description we will refer to PCM as the input format for the vocoder. Thus a vocoder includes an encoder stage that will accept as input a digitized voice signal and output a compressed signal, a possible compression ratio being 8:1. As for the reverse transformation the vocoder is provided with a decoder stage that will accept the compressed speech signal and output a digitized signal, such as PCM samples.
The main advantage of compressing speech is that it uses less of the limited available channel bandwidth for transmission. The main disadvantage is loss of speech quality.
Most modern low bit-rate vocoders are based on the linear prediction model that separates the speech signal into a set of linear prediction coefficients, a residual signal and various other parameters. Generally, the speech can be reconstructed with good quality from these components. However, degradations are introduced when speech is subjected to multiple instances of vocoders.
The rapid growth in the diversity of networks and the number of users of such networks is increasing the number of instances where two vocoders are placed in tandem to serve a single connection. In such a case, a first encoder is used to compress the speech of the first mobile user. The compressed speech is transmitted to a base station serving the local mobile where it is decompressed (converted to PCM format samples). The resulting PCM samples arrive at the base station serving the second mobile terminal, over the digital trunk of the telephone network, where a second encoder is used to compress the input signal for transmission to the second mobile terminal. A speech decoder at the second mobile terminal decompresses the received compressed speech data to synthesize the original speech signal from the first mobile terminal. A specific example of such a case may involve a call made from a wireless terminal operating according to the North American Time Division Multiplexing Access (TDMA) system to a European standard Global System for Mobile (GSM) mobile phone.
In an attempt to eliminate the condition of vocoder tandeming, a method called &lt;&lt;bypass&gt;&gt; has been proposed in the past. The basic idea behind this approach is the provision of a digital signal processor including a vocoder and a bypass mechanism that is invoked when the incoming signal is in a format compatible with the vocoder. In use, the digital signal processor associated with the first base station that receives the RF signal from a first mobile terminal determines, through signaling and control that an identical digital signal processor exists at the second base station associated with the mobile terminal at which the call is directed. The digital signal processor associated with the first base station rather than converting the compressed speech signals into PCM samples invokes the bypass mechanism and outputs the compressed speech in the transport network. The compressed speech signal, when arriving at the digital signal processor associated with the second base station is routed such as to bypass the local vocoder. Decompression of the signal occurs only at the second mobile terminal. The "bypass" approach is described in the international application serial number PCT 95CA704 dated Dec. 13, 1995. The contents of this disclosure are hereby incorporated by reference.
This solution is only valid, though, for identical vocoders. With the rapid expansion of networks, the diversity of vocoders is quickly increasing. The bypass solution is therefore useful only for a small portion of connections involving tandem vocoding.
Thus, there exists a need in the industry for devices capable of improving the voice quality during connections that may include incompatible tandemed vocoders.